To currently wire buildings for a supply of electrical mains power through the building it is common on a customized basis to have insulated cables strung through the building by a specialist electrician where each length of wire and each termination is uniquely and separately prepared and made by for instance baring at respective ends some of the insulation from a wire, inserting the thus bared raw end into a terminal junction, then tightening a screw so as to keep the wire in position.
Such a process is repeated for each length of wire and for each core for instance the active wire, the neutral wire and where appropriate the earth connection each at both ends of each wire and for each set of connections from terminal junction to terminal junction.
Any one of these joins can be a source of serious difficulty.
If any one of these joins is loose or works loose overtime and the bare wire stays close but is separated but within an arcing distance from adjacent conductors or the terminal block there becomes the high risk of arcing which will result in heating which in turn then can lead to adjacent vulnerable items starting to burn and of course the possible destruction by fire of the building.
Such wiring and joins are usually deliberately in places out of sight which means that if there is a problem it may not be readily observable and this then clearly leads to a potential serious danger. However this is the conventional system that exists for wiring of buildings.
A further difficulty is that, because any wiring by its current nature needing to be a customised process, does need specifically trained workman to accomplish the task. It is also implicitly a slow process because it is very much a manual process where a skilled worker (an electrician) estimates a size and quantity of cable to be used for each run in a building, prepares such cable, and then has to have a sufficient available stock of fittings, junctions, switches, and cables of various capacities on hand to complete a wiring job. Thousands upon thousands of buildings have been and are being wired in this way.
The usual joining method which is still widely used because it is traditional and economic involves a brass block with a number of smooth sided bores in it and for each bore a transversely aligned threaded bore through which a screw can be tightened. The end of a bared wire is inserted into a respective smooth sided bore and a metal screw is then tightened by screwing through the threaded bore to engage and clamp the wire and hold it thereby in position in bore and at the same time effect an electrical connection by simply establishing a clamping pressure either holding the wire against the conducting side of the smooth sided bore or additionally having some electrical contact through the screw. However the bore being formed of an opaque material (brass) does not allow the location of the end of the wire to be actually seen and a usual process for an electrician is that a wire is inserted and the electrician can feel that it has gone in enough to be sure that it will have a sufficient length in the bore for the screw to effectively engage and hold this.
Things do go wrong where especially for instance two or three bared wires are to be twisted together and inserted into the same bore so that it becomes a tight fit. If the feel test is deceived then there can be a dangerous situation with partial contact or worse where the wire or wires is or are loose and over time will intermittently contact with a possible result of arcing etc. Conversely a screw may simply not be tightened. There are other connectors and connection techniques but the above is a very common very economic system that is widely used.
The problems then can be seen which are that current systems do have difficulties and also incur costs that may be considered substantial.